do I go ahead and enter the property itself on the asset list in TT for 2021?
Yes.
And, in so doing, I would use the form 3115 to capture depreciation for 2019 & 2020 only?
Yes. But understand that even though the 3115 is included with the program, it is not simple by any stretch. Unless you really know what you're doing (and most who believe they do, don't) you should seek professional help.
You can also amend your 2019 and 2020 tax returns. But that presents it's own issues since amending the 2020 return will require you to manually enter the changes made to the 2019 return when you amended it.
Remember, when amending a return, you can not import from the prior year's tax return. So you'd have to manually adjust your 2020 return when amending it to show the correct depreciation taken on the amended 2019 tax return.
Either way is a beast and therefore I recommend you seek professional help. Doing things wrong can be even more costly in the future, making the cost of professional help seem like a pittance in comparison.
